Posted by
Jonathan on Tuesday, July 11, 2006 4:28:00 PM
A lot, as it turns
out. Writing in the July issue of
Scientific American (subscription required), Timothy Goldsmith notes that “We
humans customarily assume that our visual system sits atop a pinnacle of
evolutionary success.” In fact, what we
see is only a tiny subset of a much larger reality. The most startling example of this is the recent discovery that
birds can see into the ultraviolet part of the electromagnetic spectrum.
Color vision
depends on cone cells in the retina. Humans have three different kinds of cone cells, each of which is
sensitive to a different range of wavelengths of light. Our brains compare the signals sent by these
cells to produce the sensation of color. Birds, however, have four
types of cone cells, allowing them to perceive a much broader range of electromagnetic
radiation. As a result, they inhabit a
perceptual world beyond human experience.
“[I]t is difficult
– impossible, in fact – for humans to know what [birds’] perception of colors
is actually like,” writes Goldsmith. “They not only see in the near ultraviolet, but they also see colors that
we cannot even envision.”
On the positive
side, at least humans are better off than most other mammals, which only have
two cones. “The likely explanation for
this paucity is that during their early evolution in the Mesozoic (245 million
to 65 million years ago), mammals were small, secretive and nocturnal,”
Goldsmith says. “As their eyes evolved
to take advantage of the night, they became ... less dependent on color
vision.” Our primate ancestors
reacquired a third cone as a result of a chance mutation 40 million years ago,
after they adopted a diurnal lifestyle and started living in trees. This mutation proved extremely useful in
helping them find brightly colored fruit and has been passed from generation to
generation ever since.